This invention relates to a method and means for acquiring frequency bursts in a personal communications network (PCN) handset when the PCN handset is first switched on.
When a PCN handset is first switched on the local oscillator (LO) in the receiver is required to have an accuracy .+-.10 ppm or .+-.20 Hz which is sufficient to get the broadcast control channel into the IF bandwidth but not accurate enough to allow the channel estimator to work, so the Viterbi algorithm is inoperative and digital data is inaccessible. The VA requires an accuracy of the order of 100 Hz or .+-.0.05 ppm so that there is insignificant phase shift at the ends of the data bursts relative to the centre where phase is corrected by the channel estimator. Thus the very first task of the handset is to receive frequency correction bursts on the control channel in order to correct the LO frequency such that the receiver can become fully functional and receive control data relating to frequency and time-slot allocations.
The organisation of the GSM (Global Systems Mobile) frequency correction bursts is shown in FIG. 1. The broadcast control bursts are split up and multiplexed to modulate the first burst of consecutive frames on the broadcast channel frequency. The control channel contains one "F" burst in every ten. Frames are eight bursts long and last for 4.615 ms, this means that one burst in 80 is a frequency burst at regular intervals of 46.15 ms.
The broadcast channel frequency used is fixed for each base station, however the timing of the frequency used is fixed for each known at the start when the handset is switched on from cold. If the handset has been carried to a new base station area without active hand-off, even the broadcast frequency may be unknown.
The sequence of events for acquisition of the base station is:
(i) Detect the presence and timing of the frequency bursts (also the actual choice of broadcast channel in the worst case). PA1 (ii) Correct the local oscillator in the handset using one or more F bursts. PA1 (iii) Run the channel equaliser to find accurate burst and bit timing. PA1 (iv) Run the Viterbi algorithm to demodulate the synchronisation bursts to find frequency allocations etc. PA1 (i) Digital filters: a bank or perhaps 100 IIR digital filters, each tuned to a specific offset in the .+-.20 k Hz range and having a bandwidth of 200 Hz. The rise times of these filters, 1/B, would be more or less consistent with the duration of the burst (0.525 ms) and they would operate in an integrate-and-dump mode. The work load for each filter, assuming single complex poles, would be 142 complex multiplication and additions per burst making a total of 14,200 fixed point complex operations for the whole burst. PA1 (ii) Fast Fourier Transforms (FFTs): Here, if a 128-point DFT were used, the work load would be about 128Log.sub.2 (128) * 896 fixed point complex operations. PA1 i) sampling PCN r.f. signals for successive blocks of time of duration of half the duration of a frequency burst; PA1 ii) determining the first block to have a spectrum level to exceed a threshold value, PA1 iii) comparing the spectrum levels of said first block and an adjacent block during successive occurrences of the frequency burst; and PA1 iv) adjusting the timing of the two blocks until the spectrum levels of the two blocks are substantially equal. PA1 i) sampling a group of channels having different known frequencies for successive blocks of time of a duration of half the duration of a frequency burst; PA1 ii) performing a digital discrete Fourier transform (DFT) filter operation on the signal samples in each block; PA1 iii) comparing the peak spectrum level of each DFT filtered sample with a predetermined threshold value to determine the first block to have a spectrum level to exceed the threshold value; PA1 iv) comparing the spectrum levels of said first block and an adjacent block repetitively, substantially on successive occurrences of the frequency burst, to derive a differential of the spectrum levels; PA1 v) generating an error signal representing the differential of the spectrum levels; and PA1 vi) applying said error signal in a feedback loop to adjust the sampling times of said first and adjacent blocks to drive the differential of the spectrum levels to zero.
There is little choice over the form of the detector for the correction bursts which must be a matched filter and the two alternative hypotheses which the detector evaluates are the presence of random data and the presence of correction burst. The detection problem is a classical one except that the noise against which the correction burst must be compared is not white noise but spectrally-coloured data.
The presence of gaps between carrier bursts as an aid to burst location cannot be assumed since these may be filled in multipath conditions.
Ignoring the colouration of the random data, which can easily be reduced by digital filtering, the mechanisation of the matched filter could take two forms:
As the FFT approach is numerically much more efficient the invention will be described with relevance to this approach.